1. Technical Field
The present disclosure relates to the manufacture of mesoporous, crystalline nanoparticle films. Particularly, this disclosure relates to the manufacture of titania nanoparticle films for use as dye sensitized solar cells via flame deposition of metal oxide nanoparticles onto transparent conductive substrates.
2. Background
Fossil fuels currently supply the majority of energy needs in the world and are necessary to run the global economy. However, concerns over the environmental effects of fossil fuel use, as well as questions regarding the long term supply of fossil fuels have created issues with regard to the future use of fossil fuels. In order to overcome the dependence on fossil fuels, other methods, such as wind, hydroelectric and solar, are increasingly being examined for use as alternative energy generation methods. Despite the ubiquitous abundance of solar energy around the globe, converting this energy to usable electrical energy remains expensive using current photovoltaic technologies. Electricity produced using existing photovoltaic cells remain at least a factor of 2 more expensive than electricity produced from fossil fuel burning, wind, and hydroelectric sources. Additionally, efficient crystalline silicon cells are energy intensive to produce due to the high energies associated with producing molten silicon.
Mesoporous, thin-films of titanium oxide (titania, TiO2) nanoparticles have many applications including dye sensitized solar cells (DSSC) and gas-sensors. In DSSC, a thin film of porous nanocrystalline titania several micrometers in thickness serves to immobilize a photosensitive dye containing ruthenium atoms in a complex of molecular ligands and anchoring groups. Photoexcitation of the dye leads to charge separation and metal-to-ligand charge transfer, that injects electrons into the conduction band in TiO2. For gas-sensors the film acts as an atmosphere-dependent resistor, whereby the conductance across the particle film changes by the surface adsorption of an atmospheric trace species. Thin films of titania nanoparticles may be fabricated by various methods such as sol-gel spin coating, screen printing, spray pyrolysis and doctor blading.
Currently, sol-gel and spray pyrolysis are the methods of choice in synthesizing titania nanoparticles. The sol-gel method involves gelling a colloidal suspension while spray pyrolysis uses a flame, laser, or electrical discharge to turn the precursor aerosol into nanometer scale metal oxide particles. Both of these methods can make fine particles, but the resulting particle size distributions are wide when compared to the Flame Stabilized on a Rotating Surface (FSRS) process—the principal method of the present disclosure. Sol-gel can combine nanoparticle synthesis and film deposition in a single step, but the nanoparticles are generally amorphous and flame aerosol deposition processes can deposit crack free, but mesoporous films. Although the sol-gel method affords some control over the size as well as the size distribution of the particles, the particles must be dried and often need to be calcinated to obtain the desired morphology, often at the expense of the particle size due to sintering. Furthermore, dry processes may be more commercially viable because they typically require less post processing.
In addition, existing techniques employ a three-step method to produce the film. First the nanoparticles are produced by sol gel or flame spray pyrolysis. Second the particles are mixed into an ethyl cellulose paste. And third, the paste is screen printed or squeegeed onto the transparent conducting substrate. By contrast, the FSRS method is a one-step method, where the particles are produced and deposited into a thin film using a single process.
The DSSC design represents a significant paradigm shift in solar cell technology. It is considered to be an attractive alternative to existing designs because of the low cost of the materials that comprise the bulk of the cell—namely an iodine/iodide-based electrolyte, and the titania film. The platinum catalyst and dye (usually ruthenium-based) are used in such small quantities that they do not contribute significantly to the material cost of the cell. The present disclosure focuses on further lowering the costs per unit power produced by cell not only by increasing the efficiency of the cells as described above, but also by reducing the costs associated with depositing the titania film.